1998 Merc Sable: Wierd overheating
 

I'm trying to help out a buddy with a goofy overheating situation with his car. Will give ya what we got and try to keep it brief.

His water pump gave out and the car began to overheat. He pulled over immediately and shut it down. A tow was called and brought back to his place. That night, we swapped out the water pump and all seemed to be good. Then it started to overheat in a weird way.

If the heating/cooling system is off, everything is fine. If the heat is on, you'll get heat and car maintains its temp fine. Turn on the AC, and the car overheats. Turn off the AC, all returns to normal.

The radiator fan and auxiliary fan seem to be working fine so he tried flushing the radiator and replacing the thermostat. Same condition. Ran it with no thermostat, same deal. We believe that nothing is plugging the radiator and that it was bled correctly when flushed.

At one point, the cap from the coolant reservoir basically exploded off, but the thought at that time was it was a bad reservoir (which had been recently replaced). Cant remember if that was before or after the flush.

Oil nor exhaust shows any indication of blown head gasket/cracked heads.

Could there still be air trapped in the coolant system and would that cause what I've described? Would the engine be running ok with a air bubble in the coolant system without the AC but not be able to handle the additional load with an air contaminated coolant system when the AC is turned on?

Thanks in advance for any insight... we're going nuts trying to figure it out here... :rolleyes: